Keep momentary input on the widget the cursor is on

Tests across layers, as asked, and the fifth one found a defect older than
this branch: a press fired on a widget the cursor had just left, because the
frame its hover ends is a frame it still gets dispatched on, and `should_run`
only ever looked at the cursor. A button in the corner of a list therefore
clicked when the press landed anywhere else in the row.

A widget that is not under the cursor now sees a cursor with nothing
momentary in it, which settles both halves of the question at once: it is not
its press to receive, and not its press to take from the layers below.

`CursorSense` and `CursorButton` derive `Debug`, so a failure says which
sense fired rather than `left != right`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/event/manager.rs

@@ -135,6 +135,11 @@ impl<Rsc: HasEvents + 'static, E: Event> TypeEventManager<Rsc, E> {
         ));
     }
 
+    /// What this widget registered, without running any of it.
+    pub fn registered(&self, id: WidgetId) -> impl Iterator<Item = &E> {
+        self.map.get(&id).into_iter().flatten().map(|(e, _)| e)
+    }
+
     pub fn run_fn<'a>(
         &mut self,
         id: impl IdLike,

src/default/mod.rs

@@ -1,6 +1,10 @@
 use crate::prelude::*;
 use arboard::Clipboard;
-use std::{marker::PhantomData, sync::Arc, time::Instant};
+use std::{
+    marker::{PhantomData, Sized},
+    sync::Arc,
+    time::Instant,
+};
 use winit::{
     event::{Ime, WindowEvent},
     event_loop::{ActiveEventLoop, EventLoopProxy},

src/default/sense.rs

@@ -4,14 +4,14 @@ use std::{
     rc::Rc,
 };
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorButton {
     Left,
     Right,
     Middle,
 }
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub enum CursorSense {
     PressStart(CursorButton),
     Pressing(CursorButton),
@@ -52,6 +52,19 @@ impl CursorSense {
     pub fn is_dragging(&self) -> bool {
         matches!(self, CursorSense::Pressing(CursorButton::Left))
     }
+
+    /// Takes what this sense answers to out of `cursor`, so a widget below
+    /// does not also get it. Hovering takes nothing: it goes to the topmost
+    /// widget in shape, which is not a question about the input.
+    fn take(&self, cursor: &mut CursorState) {
+        match self {
+            Self::PressStart(button) | Self::Pressing(button) | Self::PressEnd(button) => {
+                *cursor.buttons.select_mut(button) = ActivationState::Off
+            }
+            Self::Scroll => cursor.scroll_delta = Vec2::ZERO,
+            Self::HoverStart | Self::Hovering | Self::HoverEnd => {}
+        }
+    }
 }
 
 #[derive(Default, Clone)]
@@ -78,6 +91,14 @@ impl CursorButtons {
         }
     }
 
+    pub fn select_mut(&mut self, button: &CursorButton) -> &mut ActivationState {
+        match button {
+            CursorButton::Left => &mut self.left,
+            CursorButton::Right => &mut self.right,
+            CursorButton::Middle => &mut self.middle,
+        }
+    }
+
     pub fn end_frame(&mut self) {
         self.left.end_frame();
         self.middle.end_frame();
@@ -164,19 +185,46 @@ impl SensorUi for UiRenderState {
         // state like thing, but local to render state, and is passed to UiRsc events so you can
         // update it there?
         let mut active = std::mem::take(&mut rsc.events_mut().get_type::<CursorSense>().active);
+        // Narrowed as it descends: a widget takes what it answers to, and
+        // what is left is what the layers below see.
+        let mut cursor = cursor;
+        let mut hovered = false;
         for layer in self.layers.indices().rev() {
-            let mut sensed = false;
+            let mut below = cursor.clone();
+            let mut hovered_here = false;
             for (id, sensor) in active.get_mut(&layer).into_flat_iter() {
                 let shape = self.active.get(id).unwrap().region;
                 let region = shape.to_px(window_size);
-                let in_shape = cursor.exists && region.contains(cursor.pos);
-                sensor.hover.update(in_shape);
-                if sensor.hover == ActivationState::Off {
+                let over = cursor.exists && region.contains(cursor.pos);
+                // Hover goes to the topmost widget in shape and no further.
+                sensor.hover.update(over && !hovered);
+                // `is_off` would be wrong here: it counts `End`, which is
+                // the one frame a hover-end handler has to run on.
+                if !over && sensor.hover == ActivationState::Off {
                     continue;
                 }
-                sensed = true;
+                hovered_here |= over;
 
-                let cursor = cursor.clone();
+                // Momentary input belongs to whatever the cursor is on. A
+                // widget it has just left still hears its hover ending, but
+                // a press landing elsewhere is neither its press to receive
+                // nor its press to take.
+                let cursor = match over {
+                    true => cursor.clone(),
+                    false => CursorState {
+                        pos: cursor.pos,
+                        exists: cursor.exists,
+                        ..Default::default()
+                    },
+                };
+
+                for senses in rsc.events_mut().get_type::<CursorSense>().registered(*id) {
+                    for sense in senses.iter() {
+                        if matches(sense, &cursor, sensor.hover) {
+                            sense.take(&mut below);
+                        }
+                    }
+                }
 
                 let data = CursorData {
                     pos: cursor.pos - region.top_left,
@@ -191,7 +239,9 @@ impl SensorUi for UiRenderState {
                 };
                 rsc.run_event::<CursorSense>(*id, data, state);
             }
-            if sensed {
+            hovered |= hovered_here;
+            cursor = below;
+            if hovered && !is_momentary(&cursor) {
                 break;
             }
         }
@@ -204,8 +254,14 @@ pub fn should_run(
     cursor: &CursorState,
     hover: ActivationState,
 ) -> Option<CursorSense> {
-    for sense in senses.iter() {
-        if match sense {
+    senses
+        .iter()
+        .find(|sense| matches(sense, cursor, hover))
+        .copied()
+}
+
+fn matches(sense: &CursorSense, cursor: &CursorState, hover: ActivationState) -> bool {
+    match sense {
         CursorSense::PressStart(button) => cursor.buttons.select(button).is_start(),
         CursorSense::Pressing(button) => cursor.buttons.select(button).is_on(),
         CursorSense::PressEnd(button) => cursor.buttons.select(button).is_end(),
@@ -213,11 +269,12 @@ pub fn should_run(
         CursorSense::Hovering => hover.is_on(),
         CursorSense::HoverEnd => hover.is_end(),
         CursorSense::Scroll => cursor.scroll_delta != Vec2::ZERO,
-        } {
-            return Some(*sense);
     }
 }
-    None
+
+/// Whether anything is happening to the cursor beyond where it rests.
+fn is_momentary(cursor: &CursorState) -> bool {
+    cursor.scroll_delta != Vec2::ZERO || cursor.buttons.iter().any(|(_, state)| !state.is_off())
 }
 
 impl ActivationState {

src/widget/position/mod.rs

@@ -4,7 +4,7 @@ mod max_size;
 mod offset;
 mod pad;
 mod scroll;
-mod set_size;
+mod sized;
 mod span;
 mod stack;
 
@@ -14,6 +14,6 @@ pub use max_size::*;
 pub use offset::*;
 pub use pad::*;
 pub use scroll::*;
-pub use set_size::*;
+pub use sized::*;
 pub use span::*;
 pub use stack::*;

src/widget/position/sized.rs

@@ -1,12 +1,12 @@
 use crate::prelude::*;
 
-pub struct SetSize {
+pub struct Sized {
     pub inner: StrongWidget,
     pub x: Option<Len>,
     pub y: Option<Len>,
 }
 
-impl SetSize {
+impl Sized {
     fn apply_to_outer(&self, ctx: &mut SizeCtx) {
         if let Some(x) = self.x {
             ctx.outer.x.select_len(x.apply_rest());
@@ -17,7 +17,7 @@ impl SetSize {
     }
 }
 
-impl Widget for SetSize {
+impl Widget for Sized {
     fn draw(&mut self, painter: &mut Painter) {
         painter.widget(&self.inner);
     }

src/widget/ptr.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::Unsize;
+use std::marker::{Sized, Unsize};
 
 pub struct WidgetPtr {
     pub inner: Option<StrongWidget>,

src/widget/text/build.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use std::marker::PhantomData;
+use std::marker::{PhantomData, Sized};
 
 pub struct TextBuilder<State, O = TextOutput, H: WidgetOption<State> = ()> {
     pub content: String,

src/widget/trait_fns.rs

@@ -31,9 +31,9 @@ widget_trait! {
         }
     }
 
-    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, SetSize> {
+    fn sized(self, size: impl Into<Size>) -> impl WidgetFn<Rsc, Sized> {
         let size = size.into();
-        move |state| SetSize {
+        move |state| Sized {
             inner: self.add_strong(state),
             x: Some(size.x),
             y: Some(size.y),
@@ -58,18 +58,18 @@ widget_trait! {
         }
     }
 
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
+    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
         let len = len.into();
-        move |state| SetSize {
+        move |state| Sized {
             inner: self.add_strong(state),
             x: Some(len),
             y: None,
         }
     }
 
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, SetSize> {
+    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
         let len = len.into();
-        move |state| SetSize {
+        move |state| Sized {
             inner: self.add_strong(state),
             x: None,
             y: Some(len),

tests/pointer_routing.rs

@@ -0,0 +1,255 @@
+//! Input across layers: what a widget takes, what passes through it, and
+//! where hovering stops. These drive `run_sensors` directly, which needs no
+//! GPU and no window.
+
+use iris::prelude::*;
+use std::{cell::RefCell, rc::Rc};
+
+struct SenseRsc {
+    ui: UiData,
+    events: EventManager<SenseRsc>,
+}
+
+impl UiRsc for SenseRsc {
+    fn ui(&self) -> &UiData {
+        &self.ui
+    }
+    fn ui_mut(&mut self) -> &mut UiData {
+        &mut self.ui
+    }
+    fn on_draw(&mut self, active: &ActiveData) {
+        self.events.draw(active);
+    }
+    fn on_undraw(&mut self, active: &ActiveData) {
+        self.events.undraw(active);
+    }
+    fn on_remove(&mut self, id: WidgetId) {
+        self.events.remove(id);
+    }
+}
+
+impl HasState for SenseRsc {
+    type State = ();
+}
+
+impl HasEvents for SenseRsc {
+    fn events(&self) -> &EventManager<Self> {
+        &self.events
+    }
+    fn events_mut(&mut self) -> &mut EventManager<Self> {
+        &mut self.events
+    }
+}
+
+const WINDOW: f32 = 100.0;
+
+/// Every sense that has fired on one widget since it was last read.
+#[derive(Default, Clone)]
+struct Fired(Rc<RefCell<Vec<CursorSense>>>);
+
+impl Fired {
+    fn take(&self) -> Vec<CursorSense> {
+        std::mem::take(&mut self.0.borrow_mut())
+    }
+}
+
+struct Ui {
+    rsc: SenseRsc,
+    render: UiRenderState,
+    state: (),
+}
+
+impl Ui {
+    fn new() -> Self {
+        Self {
+            rsc: SenseRsc {
+                ui: UiData::default(),
+                events: EventManager::default(),
+            },
+            render: UiRenderState::new(),
+            state: (),
+        }
+    }
+
+    fn listen<W: Widget + ?core::marker::Sized + 'static>(
+        &mut self,
+        widget: &StrongWidget<W>,
+        senses: impl Into<CursorSenses>,
+    ) -> Fired {
+        let fired = Fired::default();
+        let sink = fired.clone();
+        self.rsc
+            .register_event(widget.weak(), senses.into(), move |ctx, _rsc| {
+                sink.0.borrow_mut().push(ctx.data.sense)
+            });
+        fired
+    }
+
+    /// Stacks the widgets bottom first, each on its own layer, and lays them
+    /// out in a square window.
+    fn stack(&mut self, children: Vec<StrongWidget>) {
+        let root = self
+            .rsc
+            .ui
+            .widgets
+            .add_strong(Stack {
+                children,
+                size: StackSize::default(),
+            })
+            .any();
+        self.render.resize((WINDOW, WINDOW));
+        self.render.update(&root, &mut self.rsc);
+    }
+
+    fn cursor(&mut self, at: (f32, f32)) -> CursorState {
+        CursorState {
+            pos: at.into(),
+            exists: true,
+            buttons: Default::default(),
+            scroll_delta: Vec2::ZERO,
+        }
+    }
+
+    fn run(&mut self, cursor: CursorState) {
+        self.render.run_sensors(
+            &mut self.rsc,
+            &mut self.state,
+            cursor,
+            (WINDOW, WINDOW).into(),
+        );
+    }
+}
+
+fn full(ui: &mut Ui) -> StrongWidget<Rect> {
+    rect(UiColor::WHITE).add_strong(&mut ui.rsc)
+}
+
+#[test]
+fn hover_stops_at_the_topmost_widget() {
+    let mut ui = Ui::new();
+    let (bottom, middle, top) = (full(&mut ui), full(&mut ui), full(&mut ui));
+    let bottom_hover = ui.listen(&bottom, CursorSense::HoverStart);
+    let middle_hover = ui.listen(&middle, CursorSense::HoverStart);
+    let top_hover = ui.listen(&top, CursorSense::HoverStart);
+    ui.stack(vec![bottom.any(), middle.any(), top.any()]);
+
+    let cursor = ui.cursor((50.0, 50.0));
+    ui.run(cursor);
+
+    assert_eq!(top_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        middle_hover.take(),
+        [],
+        "hover is not shared with a layer below"
+    );
+    assert_eq!(bottom_hover.take(), []);
+}
+
+#[test]
+fn a_scroll_passes_through_every_widget_that_does_not_want_it() {
+    let mut ui = Ui::new();
+    let (list, button, overlay) = (full(&mut ui), full(&mut ui), full(&mut ui));
+    let scrolled = ui.listen(&list, CursorSense::Scroll);
+    let clicked = ui.listen(&button, CursorSense::click());
+    let overlay_clicked = ui.listen(&overlay, CursorSense::click());
+    ui.stack(vec![list.any(), button.any(), overlay.any()]);
+
+    let mut cursor = ui.cursor((50.0, 50.0));
+    cursor.scroll_delta = (0.0, 10.0).into();
+    ui.run(cursor);
+
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "two layers of click-only widgets do not stop a scroll"
+    );
+    assert_eq!(clicked.take(), []);
+    assert_eq!(overlay_clicked.take(), []);
+}
+
+#[test]
+fn a_click_and_a_scroll_in_one_frame_go_to_different_widgets() {
+    let mut ui = Ui::new();
+    let (list, button) = (full(&mut ui), full(&mut ui));
+    let scrolled = ui.listen(&list, CursorSense::Scroll);
+    let clicked = ui.listen(&button, CursorSense::click());
+    ui.stack(vec![list.any(), button.any()]);
+
+    let mut cursor = ui.cursor((50.0, 50.0));
+    cursor.scroll_delta = (0.0, 10.0).into();
+    cursor.buttons.left = ActivationState::Start;
+    ui.run(cursor);
+
+    assert_eq!(clicked.take(), [CursorSense::click()]);
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "taking the click must not take the scroll with it"
+    );
+}
+
+#[test]
+fn only_the_topmost_listener_takes_a_press() {
+    let mut ui = Ui::new();
+    let (below, above) = (full(&mut ui), full(&mut ui));
+    let below_clicked = ui.listen(&below, CursorSense::click());
+    let above_clicked = ui.listen(&above, CursorSense::click());
+    ui.stack(vec![below.any(), above.any()]);
+
+    let mut cursor = ui.cursor((50.0, 50.0));
+    cursor.buttons.left = ActivationState::Start;
+    ui.run(cursor);
+
+    assert_eq!(above_clicked.take(), [CursorSense::click()]);
+    assert_eq!(below_clicked.take(), [], "one press goes to one widget");
+}
+
+#[test]
+fn a_press_beside_the_button_reaches_the_layer_below() {
+    let mut ui = Ui::new();
+    let list = full(&mut ui);
+    // The row above the list covers it, but only its left half is the button.
+    let button = full(&mut ui);
+    let gap = full(&mut ui);
+    let list_clicked = ui.listen(&list, CursorSense::click());
+    let button_clicked = ui.listen(&button, CursorSense::click());
+    let row = ui.rsc.ui.widgets.add_strong(Span {
+        children: vec![button.any(), gap.any()],
+        dir: Dir::RIGHT,
+        gap: 0.0,
+    });
+    ui.stack(vec![list.any(), row.any()]);
+
+    let mut on_button = ui.cursor((20.0, 50.0));
+    on_button.buttons.left = ActivationState::Start;
+    ui.run(on_button);
+    assert_eq!(button_clicked.take(), [CursorSense::click()]);
+    assert_eq!(list_clicked.take(), []);
+
+    let mut beside_it = ui.cursor((80.0, 50.0));
+    beside_it.buttons.left = ActivationState::Start;
+    ui.run(beside_it);
+    assert_eq!(button_clicked.take(), [], "the cursor is not on the button");
+    assert_eq!(
+        list_clicked.take(),
+        [CursorSense::click()],
+        "a press beside the button belongs to what is under it"
+    );
+}
+
+#[test]
+fn leaving_a_widget_still_ends_its_hover() {
+    let mut ui = Ui::new();
+    let widget = full(&mut ui);
+    let hover = ui.listen(&widget, CursorSense::HoverStart | CursorSense::HoverEnd);
+    ui.stack(vec![widget.any()]);
+
+    let cursor = ui.cursor((50.0, 50.0));
+    ui.run(cursor);
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    let mut gone = ui.cursor((50.0, 50.0));
+    gone.exists = false;
+    ui.run(gone);
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+}